mfd: stmpe: DT: Enable no-irq mode configuration

If there is no interrupt property into stmpe node
then activate the no-irq mode by setting the irq
value to -1.

Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>